In order to bring such a selective permeation and separation process of oxygen, or partition membrane reactor, and the like into a practical use, materials having high oxide ion conductivity are required. As a material to satisfy the requirement, mixed conducting oxide having a perovskite structure is being studied. The perovskite structure is a crystal structure in which cations occupy A site where 12 pieces of anionic oxygen are coordinated and B site where 6 pieces of oxygen are coordinated respectively. Many of the materials studied for the above-described object contain Co or Fe in B site.
For instance, ceramic compositions such as (LaxSr1-x)CoO3-α, (x is in the range of 0.1 to 0.9, α is 0 to 0.5), which is disclosed in Japanese Patent Application Laid-open No. Sho 56-92103, (La1-xSrx)(Co1-yFey)O3-δ (x is in the range of 0.1 to 1.0, y is 0.05 to 1.0, δ is 0.5 to 0), which is disclosed in Japanese Patent Application Laid-open No. Sho 61-21717, and so on are known as a useful leading top-rated materials. Further, in Japanese Patent Application Laid-open No. Hei 6-206706, proposed is an oxide ion transfer permeable membrane having an extremely wide composition range composed of AxBax·ByB′y·B″y·O3-z (A is selected from a group consisting of a first, second, and third families in a periodic table and a lanthanoid family of f period adopted by ICUPA, and B, B′, and B″ are selected from transition metals of d period. Further, the following conditions of 0≦x≦1, 0<x′≦1, 0<y≦1, 0≦y′≦1, 0≦y″≦1, x+x′=1, y+y′+y″=1, are satisfied and z is a value determined when electric charge of the composition is neutral). As the concrete examples for the above, La0.2Ba0.8Co0.8Fe0.2O2.6, and the like are given.
Y. Teraoka et al. studied an oxygen permeation rate of perovskite structure oxides expressed by a composition formula of La0.6A′0.4Co0.8Fe0.2O3-δ and pointed out in Chemistry Letters, pp. 503–506, 1988 that the oxygen permeation rate could be improved by containing Ba in A site of the perovskite structure. When considering this knowledge expandedly, it can be expected that improvement of the oxygen permeation rate of the perovskite structure mixed conductor oxides can be realized by substituting Sr or La which are often used as an element placed in A site of the perovskite structure oxides of mixed conductors for Ba as mush as possible. Especially, substitution of La having a valence of 3 for Ba having a valence of 2 leads to increase of oxygen holes in a crystal which is a carrier of oxygen permeation, and an effect of “killing two birds with one stone” is expected as a measure for improvement of the oxygen permeation rate.
However, as shown in “Perovskite related compounds” Kikan Kagaku sosetsu, No.32 (1997), pp. 11–13, edited by Chemical Society of Japan, it is known that when A site of perovskite is replaced from Sr having a small ion radius to Ba having a large ion radius, a structure having a BaNiO3 type or the like becomes more stable than the perovskite structure and easier to appear. It is shown by an actual preliminary experiment carried out by the inventors that though the crystals structure of a sintered body of SrCo0.8Fe0.2O3-δ was a cubic perovskite structure, in BaCo0.8Fe0.2O3-δ, a phase of low oxygen permeation rate different in crystal structure from the perovskite structure was found. This different phase is belonged to a hexagonal 12H-BaCoO3-x type structure, reported by A. J. Jacobson et al. in J. Solid State Chemistry, vol.35 (1980) pp.334–340, which is similar structure to BaNiO3 type.
Similarly, a La0.2Sr0.8CoO3-δ sintered body has a cubic perovskite structure while La0.2Ba0.8CoO3-δ has fallen into a 12H-BaCoO3-x type structure. Conventionally, Y has been recognized as an element to substitute for La in A site of the perovskite as Masannek et al. disclosed in Japanese Patent Application Laid-open No. Hei 6-56428. However, as a result that the present inventors synthesized a composition containing Y in A site and having a large ratio of Ba, such as Ba0.8Sr0.1Y0.1CoO3-δ, and studied the structure, a stable perovskite structure could not be obtained. H. W. Brinkman studied BaCo0.95Y0.05O3-δ which substituted Y in B site, and reported that the crystal structure was BaCoO3-x type hexagonal as expected, in Solid State Ionics, Vol.68(1994)PP.173–176. As described above, substitution by Ba in A site was reconfirmed as a factor to make the perovskite structure unstable.
As mentioned above, different phase such as BaNiO3 type or its analogous structure is extremely low in oxygen permeation rate, materials showing these phases cannot be used in an oxygen-separation apparatus or the like. In other words, when the oxygen permeation rate is intended to be improved by increasing Ba ratio by substituting Ba for La or Sr in A site in conventional mixed conducting oxide material such as (La1-xSrx)(Co1-yFey)O3-δ, it used to have a dilemma that stability of a cubic perovskite structure becomes insufficient, and a different phase such as BaNiO3 type is appeared to make the oxygen permeation rate rather low.